A photoacoustic gas sensor may be used to detect a concentration level of a gas component in a gas sample. In photoacoustic gas sensors, a gas mixture is illuminated with light that has a certain wavelength or range of wavelengths. A target gas component of the gas mixture absorbs the light and increases the temperature of the gas mixture. The increase in temperature of the gas mixture increases the pressure of the gas mixture. The increase in pressure of the gas mixture due to the temperature increase can be measured. A microphone can be used to detect and measure the increase in pressure of the gas mixture.
A higher concentration of the target gas component in the gas mixture will cause a higher increase in the pressure of the gas mixture. This higher increase in the pressure of the gas mixture causes the detecting microphone to record a louder sound.
A conventional photoacoustic gas sensor 100 in resonance mode is illustrated in FIG. 1. A light source 110 (such as a light bulb or laser) generates modulated light 120 of a certain wavelength. The amplitude of the light is modulated, e.g., with a square wave with ON and OFF states. The light 120 enters a photoacoustic cell 130 (also referred to as a photoacoustic gas sensor chamber 130) through a transparent or translucent wall 140. The light amplitude modulation frequency should be the same as the resonance frequency of the sound wave in the photoacoustic cell 130.
The opposite wall of the photoacoustic cell 130 comprises an opaque wall 150 or a mirror 150. In the embodiment shown in FIG. 1, the photoacoustic cell 130 is in the shape of a cylinder. A microphone 160 is associated with and is in acoustic contact with the interior of the photoacoustic cell 130. The microphone 160 detects the pressure within the photoacoustic cell 130.
It is important that the light illuminate only a portion (and not all) of the photoacoustic cell 130. The resonance frequency corresponds to a standing wave with nodes and anti-nodes of the sound pressure. The nodes are surfaces within the cell 130 and the anti-nodes are the spaces between the surfaces. The pressure at the nodes is constant, and the pressure in an anti-node at a given time is either increasing or decreasing (except for two points during the cycle when the pressure is not changing at any point).
For a cylindrically shaped cell 130 of the type that is shown in FIG. 1 there will be a standing sound wave along the symmetry axis. The pressure node 195 is the plane in the middle of the cell 130 between the two flat ends of the cell 130. One anti-node is located to the right of the pressure node 195 in the cell 130. The other anti-node is located to the left of the pressure node 195. The modulated light should illuminate only an anti-node if the pressure is increasing to introduce more energy into the standing wave and make the pressure variations greater.
For a cylindrically shaped cell 130 of the type that is shown in FIG. 1 this means that only one half of the cell 130 is illuminated with modulated light. During the time when the modulated light is on, the pressure in the illuminated portion of the cell 130 increases. During the time when the modulated light is off, the pressure in the previously illuminated portion of the cell 130 decreases. The other (non-illuminated) portion of the cell 130 is present to complete the resonance space of the cell 130.
The output of the microphone 160 is provided to an amplifier 170. The amplifier 170 amplifies the output signal of the microphone 160 and provides the amplified output to an analog-to-digital converter 180. The analog-to-digital converter 180 converts the amplified analog signal to a digital signal and provides the digital signal to a controller 190. The modulation signal of the light source 110 is either generated in the controller 190 and sent to the light source 110 or generated near the light source 110 and sent to the controller 190. The controller 190 uses the digital signal and the modulation signal to determine the concentration level of the target gas component that is in the mixture of gases within the photoacoustic cell 130.
In the analysis of a gas mixture, it is often useful to have many independent measurements of the gas mixture. It is also often useful to determine a concentration of a second target gas component in a mixture of gases in which the concentration of a first target gas component has already been measured. In conventional photoacoustic gas sensors, a concentration of a second target gas component is determined using a second light source that generates light with a wavelength that is absorbed by the second target gas component. This technique requires the use of a second light source.